This invention relates to an AC drive type electroluminescent display device.
Various types of the display devices of the type described above have been developed for many years, but they still involve many problems to be solved. To obviate these difficulties, a display device utilizing an AC drive type electroluminescent display device has recently been developed capable of increasing the number of picture elements as well as the density, thereby preventing flickering and decrease in the brightness. Usually, the electroluminescent display device comprises a plurality of electroluminescent elements arranged in a matrix on a transparent substrate and transistors for driving the electroluminescent elements are provided corresponding to respective elements. Transistors corresponding to the electroluminescent elements to be operated are selectively driven to cause the elements to luminesce for displaying desired patterns, digits or letters. The transistors for driving the electroluminescent elements are required to be formed on a transparent substrate such as a glass panel and to have a high breakdown voltage characteristic. For this reason, thin film transistors utilizing as their semiconductor layers such binary compounds as CdSe, CdS, etc. have been generally used. Such transistors are described, for example in T. P. Brody et al paper entitled "A 6.times.6-in 20-lpi Electroluminscent Display Panel; I.E.E.E. Transactions ON Electron Device, Vol. ED-22, No. 9, September 1975.
Thin film transistors utilizing such binary compounds have a high mobility of the carriers, and a high breakdown voltage characteristic so that they are suitable for use in the display device of the type described above, but still accompany various problems as follows.
The binary compound semiconductors utilized in the thin film transistors lack stability of the characteristic, reliability over a long period, and the uniformity of the characteristics of the substrate. These factors cause a short life of the resulting thin film transistors. To obviate these difficulties, it has been proposed to perform all manufacturing steps in vacuum. Even with such measure, however, the characteristics of the thin film transistors vary with time. Typical examples of such characteristics are shown in FIG. 4. At present, it is considered that the instability of the characteristics of the binary compound semiconductors is caused by their stoichiometric compositions.
Furthermore, the binary compound semiconductors can not form insulating films by oxidation reaction. Accordingly, a gate insulating film, for example, is formed by sputtering or vapor-depositing of the oxide film of other elements, for instance SiO.sub.2 or Al.sub.2 O.sub.3. According to this method, however, since a material different from semiconductor layer material is vapor-deposited on the semiconductor layer, the interface is not uniform with the result that numerous trap levels are formed at the interface which trap electric charge, thus decreasing drain current, and degrading the characteristic of the interface between the gate insulating film and the semiconductor layer. This also degrades the reproduceability and uniformness of the element characteristics of the thin film transistors. To form a thin film transistor by using a binary compound semiconductor, a vacuum evaporation mask is used so that the accuracy of mask aligning is about 100 microns so that the width of a pattern formed through a vacuum evaporation mask is about 30 microns and the minimum width of the pattern becomes more than 30 microns. Therefore, it is impossible to increase the picture element density to a value higher than one line/mm when the transistors are combined with electroluminescent elements to fabricate a display device, thus making it difficult to obtain the display device of high density.